DIO1 and DIO2 will boot up as inputs, assuming factory default settings for each of these registers, so I suppose you could ground these pins, without consequence, as long as you do not configure these registers otherwise.

Yes, you can leave them floating. Just to be clear, when I say, "floating," that means that they are not connected to anything, except the pad on the PCB and perhaps a small trace to a nearby test point. We have seen cases where some have presumed that they can connect these pins to long cables, "just in case" they want to use them at a later time, as long as they are not terminated at the other end of the cables. In a couple of those cases, noise on the ~RST line ended up causing problems during the start-up process. I am not aware of any such sensitivities on the DIOx or AUX_ADC pins, but I would avoid any unnecessary risk of exposure to noise or other threats. In the spirit of "best engineering practice," we always encourage implementations and design practices that are "only as complex as necessary" to assure proper power and communication service to/from the ADIS16209 (and devices like it).

You probably already do this, but we do have to write these responses with the assumption that other people will use them in the future, so please forgive me for this expansion in my response. I hope that this helps!